I looked up the numbers for Ivo Karlovic, the 6'10", 31 year old Croatian. I saw him play Roddick on clay in 2003 - unbelievable serving both ways. Roddick won, but later said that Karlovic seemed to be "serving out of a tree." Karlovic's current stats show that he wins 91% of his service games, and only 10% of return games. I put him in that second tier also.